The present invention relates to a composition comprising at least one rubber, said composition being intended, in particular, to serve as connecting rubber for the manufacture of a new tire, or as connecting rubber or repair rubber for the recapping or repair of a worn tire.
The invention also concerns a process which consists in vulcanizing said composition, for instance in order to assure the connection between two rubber parts, in particular upon the manufacture, recapping or repair of a tire.
Two main recapping processes are known, namely:                the first process, known as “mold recapping” or “hot recapping”, consists in placing a new raw tread, free of tread pattern, on the used carcass after it has been decapped and brushed, and curing the tire assembly thus formed in a mold, under pressure, at temperatures of about 150° C.; the tread pattern is imparted upon this operation by the action of the hot mold under pressure;        the second process is based on the concept of “cold vulcanization”, that is to say vulcanization at a relatively low temperature of between 95 and 140° C. and, in particular, between 95 and 115° C.        
This second cold vulcanization process is described, for instance, in U.S. Pat. Nos. 3,951,720, 2,976,910, 3,136,673, and 3,236,709, and in “Tire Technology International 1993” (The Annual Review of Tire Materials and Tire Manufacturing Technology, UK & International Press, 1993, p. 194).
In this process, a raw layer of a rubber preparation is interposed between the decapped tire and the precured replacement tread, this layer being intended to assure the connection of the materials after curing at low temperature. Such rubber preparations are thus commonly referred to in the art as “connecting rubbers”. As the carcass and the replacement tread are already vulcanized, the recuring temperature upon the recapping should be as low as possible in order not to impair the performance of the tire, as mentioned in the reference “La chaleur raccourcit la vie du pneumatique” [Heat shortens the life of tires] (Bandag, Communication Pneu, 1992, p. 32).
In this cold vulcanization process, good adherence of the connecting rubber to the precured adjacent mixes requires a high degree of vulcanization. The high coherence necessary for the connecting rubber is acquired by the cross-linking upon the low temperature curing. For these connecting rubbers it is therefore necessary to use vulcanization systems which are very reactive at low temperature so as to assure fast curing speeds and high degrees of cross-linking. These two performances of the vulcanization system are indispensable in order to improve the productivity and quality of this type of recapping.
However, it is also indispensable that these vulcanization systems do not result in early vulcanization in the normal conservation conditions of the connecting rubbers at the temperatures at which they are normally stored or transported. Vulcanization systems which are very reactive on curing are frequently unstable in raw state. A solution known in the art for reconciling the requirements of reactivity and of stability of these vulcanization systems consists in keeping the vulcanization agent, on the one hand, and the vulcanization accelerators, on the other hand, separate during storage and in permitting them to come into contact with each other only during the recapping operation. Thus, for example, U.S. Pat. No. 2,976,910 discloses the joint use of a solution containing either the cross-linking agent or the vulcanization accelerators and of a film which contains the missing ingredients of the vulcanization system. It is also possible, as disclosed in U.S. Pat. No. 3,136,673, to use two films, one containing the cross-linking agent and the other the vulcanization accelerators.
On the other hand, new constraints of a toxicological nature have appeared in recent years based on the strict European regulations which limit the amount of certain nitrosamines in the storage areas and curing shops to very low values.
In Germany, for instance, there exists the “Technical Rule for Dangerous Substances”, TRGS No. 552, which is devoted specifically to nitrosamines and establishes maximum concentrations for the latter in the air.
TRGS No. 552 is cited in several technical articles which point out the carcinogenic effect of nitrosamines and propose possible substitutes for the products which are precursors of carcinogenic nitrosamines:                “A new safe thiuram, TBzTD” (D. B. Seeberger, 136th Meeting of the Rubber Division, ACS Paper No. 70, 1989).        “Les nitrosamines: un défi bien réel” [Nitrosamines: A real challenge] (H. W. Engels, Caoutchoucs & Plastiques, No. 715, pages 52 to 57, 1992).        “Les nitrosamines dans l'industrie du caoutchouc: le point sur la question en République Fédérale d'Allemagne” [Nitrosamines in the rubber industry: The focus on the matter in the Federal Republic of Germany] (P. Lüpfert, Kautschuk Gummi, Kunststoffe, Vol 42, No. 1, pages 16 to 21, 1989).        “N-nitrosamines volatiles et atmosphères industrielles” [Volatile N-nitrosamines and industrial atmospheres] (N. Daubourg, A. Coupard and A. Pepe, Caoutchoucs et Plastiques, No. 717, pages 103 to 11, October, 1992).        
In the tire industry, vulcanization accelerators are very frequently precursors of nitrosamines; now, there are primarily twelve nitrosamines which are classified as carcinogenic and are cited in TRGS No. 55, such as N-nitrosomorpholine, N,N-dimethylnitrosamine, N,N-diethylnitrosamine, and these nitrosamines are for the most part capable of being produced by nitrosation of the decomposition products of the vulcanization accelerators.
In this context of industrial hygiene, it is therefore important that the systems for the vulcanization of rubber compositions in general and the connecting rubbers in particular be without carcinogenic nitrosamine precursors which are covered by the regulations.
The rubber compositions which are intended in particular for use as connecting rubbers and which contain complete vulcanization systems, that is to say both vulcanization agents and vulcanization accelerators, must therefore at the same time present rapid curings at low temperature with high yields, excellent raw stability under the ordinary conditions of preservation, storage and transportation, and be free of any compound or ingredient which is a precursor of carcinogenic nitrosamines covered by the regulations.
In the literature, complete sulfur vulcanization systems for low-temperature curing are widely cited.
It is thus known that combinations formed of ultra-accelerators of the thiuram or dithiocarbamate family and derivatives of benzothiazole confer a high reactivity at low temperature on the mixes, as cited in:                “Room Temperature Curative Systems for Natural Rubber” (S. W. SIN, Proceedings of Technology Seminar, Dec. 4-5, 1978, pages 178 to 186).        “Vulcanization and Vulcanizing agents” (W. Hofmann, Palmerton Publishing, New York, 1967, page 139).        Encyclopedia of Chemical Technology, 3rd edition (Vol 20, published by John Wiley & Sons, Inc., 1982, pages 350 to 352).        
Moreover, it is also known that such combinations may make it possible to obtain high vulcanization yields as cited by T. D. Skinner and A. A. Watson (Rubber Age 99 (11), p. 76, 1967).
Nevertheless, the high reactivity of these products upon the curing of the compositions which contain them generally produces a marked tendency to prevulcanization and scorching at moderate temperatures which may correspond to the temperatures of preservation or storage of these compositions, as cited in “Prevulcanization inhibitor of sulfur-free thiuram vulcanization” (V. Duchacek, Rubber Chem. & Technol., 46, pages 504 to 510, 1973).
Furthermore, many thiurams and dithiocarbamates which make it possible to effect low-temperature curing are precursors of carcinogenic nitrosamines in accordance with European law such as, for instance, methyl zimate (zinc dimethyldithiocarbamate) cited in U.S. Pat. No. 3,951,720, this product being a precursor of dimethylnitrosamine which is classified as carcinogenic in Regulation TRGS 552.
Moreover, complete vulcanization systems conferring good resistance to scorching and excellent raw stability on raw rubber compositions are also known. Examples of such systems are cited in:                Encyclopedia of Chemical Technology (3rd edition, Vol. 20, published by John Wiley & Sons, Inc., 1982, pages 390 to 392).        OTOS/MBT Derivative Vulcanization Systems” (K. C. Moore, Elastomerics, 110 (6), 1978, pages 36 to 42).        
However, the modest reactivity of these known systems results in slow curing at low temperature.
Thus, in the present state of the art, there is no complete vulcanization system which is a non-precursor of nitrosamines which are carcinogenic and therefore regulated, which is capable of conferring in rubber preparations, and in particular in rubber compositions for connecting rubbers, at the same time:                rapid curing at low temperature,        a high cross-linking yield after curing at low temperature,        good stability to storage at moderate temperature (20-40° C.).        